Central venous pressure is defined as the pressure of blood in the central venous system which is essentially just prior to entry into the right atrium.
This is the point where the superior vena cava meets the inferior vena cava prior to entry into the right atrium of the heart. It is usually measured by placing a catheter in one of the veins and then threading it to this point. This procedure is usually done under aseptic techniques in a ward, operating theatre, or the intensive care unit of a well equipped and staffed hospital. A qualified medical practitioner is authorised to perform this procedure.
In normal health the amount of blood coming to the heart is balanced precisely by the amount of blood leaving the heart. The cardiovascular system will be viewed as a closed system for the purposed of this discussion. When there is blood loss or other alteration in the cardiovascular system’s homeostatic mechanism it will first be reflected in changes of the venous pressure at this point. As a rule of thumb the central venous pressure is a good indicator of the amount of blood returning to the heart from the systemic circulation. Further, it (CVP) is a good indicator of the pumping ability of the right atrium and the right ventricle. When the right atrium or ventricle is failing (e.g. following right atrial or ventricular MI) the CVP will be one of the first indicators to rise. The rising CVP indicates that the atria and/or ventricle are failing.
Diagram 1. Major blood vessels of the Heart showing blood flow.
Diagnoses of right ventricular failure and/or hypo volaemia are difficult without the use of invasive procedures. Sometimes, indirect means are used to evaluate hypo volaemia. The patient may be given a fluid challenge of 250 to 500 ml. This will cause the CVP to rise. CVP rise which is not sustained for more than 10 minutes suggests hypo volaemia. Serial readings are of greater use clinically because they more accurately predict the trend than single readings.
Normal CVP values.
In homeostasis CVP is from 0 to 8 cm of water. The Hospital where I work normally accepts 5 to 10 cm of water as normal. CVP is usually measured in cm of water but in some institutions it is measured in mm of mercury; when using the mercury scale 2 to 6 mm of mercury is considered normal.
Conditions which cause the CVP to rise.
Several conditions can cause the CVP to rise. Some situations which cause the CVP to rise are as follows: